C. GENERAL PHYSICS. 187 



such operation an autopsy of the body should be made and 

 a chemical examination had of its principal organs, so as to 

 ascertain the presence or absence of poison. But these ex- 

 aminations, which have value only when they are conducted 

 in a truly scientific manner, are always delicate, even when 

 the field of investigation has been limited by judicial instruc- 

 tions ; and they would become extremely long and trouble- 

 some in the absence of any preliminary indication. More- 

 over, even if it be admitted that they would be conducted 

 with the necessary care and skill so long as the number of 

 cremations is small, it by no means follows that this would 

 be the case when the demands for incineration multiplied. 

 Annales de Chimie et de Physique^Y.^ viii., Avgiist, 187G, 571. 



THE DIETIIEROSCOPE. 



Professor Luvini, of Turin, exhibited at the Scientific Loan 

 Exhibition in London, and also at the International Exposi- 

 tion at Philadelphia, an instrument on a totally new princi- 

 ple for measuring atmospheric refraction. Its construction 

 is as follows : If one half of a lens be covered, the image 

 produced by it will difier only in brightness from that formed 

 by the uncovered lens. Moreover, if we take two lenses of 

 unequal focal length, and place them at a distance from each 

 other equal to the sum of their focal lengths, the rays emerg- 

 inir from the second will have the same desrree of convero^ence 

 as those entering the first; i. e., the object looked at will ap- 

 pear in its natural size and position. Any agency such as 

 irregular refraction of the atmosphere which alters the path 

 of the light from the object to the system of lenses will alter 

 the position of the image formed. If now we have a tele- 

 scope of such a size that the lenses of the dietheroscope cover 

 half of the aperture of the object-glass, two images of the 

 distant object can be formed one as seen through the diethe- 

 roscope, the other as seen beside it ; the latter image being 

 formed by the rays coming directly from the object to the 

 telescope. If the telescope is astronomical the latter image 

 will be reversed, while that transmitted through the diethe- 

 roscope will be in its natural position. The distance be- 

 tween these two images will depend upon the refraction of 

 the atmosphere, and so the instrument may be used to meas- 

 ure that refraction. Professor Luvini proposes that four of 



